PTC

Positive Temperature Coefficient or also PTC thermistor. PTC thermistors are semi-conductor resistors that are temperature-dependent.  PTC thermistors have a positive temperature coefficient. They consist of electrically conductive materials that can conduct electricity better at lower temperatures than at high temperatures.  Their electrical resistance increases with increasing temperature.

Resistance increases approximately linearly with the temperature

In principal, all metals have a positive temperature coefficient. In this kind of semi-conductor, the lattice arrangement of the atoms results in one free valance electron per atom. These atoms are easily movable. Connected to a power source, the free valance electrons move to the positive pole and cause the electrical conductivity. Almost all metals are PTC thermistors, as they conduct better at lower temperatures than at high temperatures. PTCs consist of polycrystalline titanate ceramic varieties that are contaminated with foreign atoms (doping).
 
The PTC-conductors are used as temperature sensors for mechanical stick guns. Tank systems use PT-100 temperature sensors based on platinum.